Identification of a chemical substance is often carried out using a combination of separation and analysis. Separation of a liquid analyte into its different components is commonly carried out using liquid chromatography (LC) or capillary electrophoresis (CE). To minimise ion fragmentation, analysis is carried out by first ionizing the liquid at atmospheric pressure using an electrospray ionization (ESI) source. However, analysis typically takes place under vacuum, using a mass spectrometer (MS). The ions, which normally comprise a small fraction of an entraining gas flow, must therefore be coupled between regions at atmospheric pressure and at low pressure. Coupling between the two pressure regimes is carried out in an intermediate pressure chamber known as a vacuum interface.
Efficient vacuum interfaces subject the gas and ion flow to an adiabatic compression-expansion process, of the type originally developed for the production of cluster beams [Kistiakowsky 1951; Deckers 1963; Campargue 1964; U.S. Pat. No. 3,583,633] and known as a free jet expansion. Such systems were later adapted to ESI-MS systems [Yamashita 1984; Bruins 1987; U.S. Pat. No. 453,056]. In any such process, supersonic velocities can be achieved, effectively by trading the thermal energy of ions and molecules for kinetic energy in the forward direction. As a result, the flow becomes collimated, allowing a considerable improvement in coupling efficiency into any downstream analysis device such as a mass filter.
A common method of free jet expansion involves expansion of the flow into an intermediate pressure chamber. The gas entering the chamber forms a barrel-shaped volume known as a shock bottle, bounded by oblique shock waves, at the end of which is located a normal shock known a Mach disc. Experiments have shown that, if the Mach disc can be punctured using a sharp conical metal skimmer, the flow through the skimmer orifice can undergo further shock-free expansion into a low-pressure chamber and hence remains collimated.
A key requirement is the ability to construct intermediate chambers with suitable input orifices and skimmer cones. Large metal skimmer cones can be fabricated using conventional machining. Smaller cones can be formed by electroplating layers of metal on the outside of a suitably shaped mandrel, machining away the tip to form an orifice, and detaching the electroplated structure using thermal shock [Gentry 1975]. The cone may then be attached to a bulkhead between the intermediate and low-pressure chambers. However, as systems become miniaturised, it becomes increasingly difficult to form suitable skimmer components with sufficient precision. Microfabrication methods such as electro-discharge-machining (EDM) may be used for the initial shaping [Kuo 1992]. Tapered skimmers with microscopic orifices may be constructed from melted and stretched silica capillaries [Grams 2006]. However, these methods yield discrete components that require alignment and attachment to pressure bulkheads.
In alternative applications, miniature nozzle components have been fabricated by etching pyramidal shaped holes in silicon substrates using anisotropic wet chemical etching [Mukherjee 2000]. However, the application was a microthruster, and cone-shaped skimmers were not formed. Microfabricated nozzles have also been fabricated by first etching a stepped hole in a silicon substrate by deep reactive ion etching (DRIE), forming a layer of silicon dioxide, and partly removing the silicon to reveal the silicon dioxide [Wang 2007]. However, the application was an electrospray source and smooth tapered features were not formed.
Vacuum interface components have been also formed in silicon. U.S. Pat. No. 7,786,434 described a silicon-based vacuum interface, formed by structuring silicon using plasma-based deep reactive ion etching (DRIE) and stacking etched dies together to form a complete intermediate chamber with aligned entrance and exit orifices. Similar components have been incorporated into miniature ESI-MS systems [Wright 2010; Malcolm 2011]. However, the design lacked a suitably shaped skimmer cone and instead used an etched capillary outlet, leading to a significant reduction in useful ion coupling efficiency. U.S. Pat. No. 7,922,920 described a similar interface component formed from stacked silicon dies, incorporating meandered input channels but again lacking a skimmer cone.
Accordingly there is a need to develop new methods capable of combining miniature skimmer cones with the other components needed to construct complete miniature vacuum interfaces capable of providing shock-free supersonic expansions.